I have 2 1998 V70 wagons. One has 230K miles, the other 210K. On both of them, the AC works (though the one with lower miles has a slow leak in the system). However, both of them have started making a clicking noise that seems to be coming from somewhere behind the dashboard. I do not hear it from the front of the car if I look under the hood, only from inside the passenger compartment. The A/C pressure tester indicates the middle of the "correct" level of charge. I was thinking of adding some 134a, but I do not want to overcharge the system. (Does overcharging the system hurt it?) Any thoughts? Is it just old age noise, or should I look into something specific?